A bonding wire used in a manufacturing process of semiconductor devices has been composed of a conductive metal core wire material of, for example, gold (Au), copper (Cu), aluminum (Al) and the like in the form of a bare wire.
Therefore, when bonding pads of a semiconductor pellet are bounded to conductors to external output terminals such as inner leads of a lead frame using such wires, a problem of short-circuiting arises in such a case that a wire comes into contact with another wire, an inner lead, a tab or the like due to any reason such as poor bonding, resin flow in a resin mold type package or the like.
In particular, as a distance between a pellet and inner leads is increased in correspondence to the increase in the number of pins in a large scale integrated circuit (LSI), a length of a wire span must be increased. In this case, however, short-circuiting, faulty chip edges and mount touches and the like are more liable to be caused by a wire which is curled and comes into contact with an adjacent wire.
Consequently, it has been proposed to coat a bonding wire with an insulation film. For example, U.S. patent application Ser. No. 4,488,674 proposes to coat a bonding wire with a polymer resin material which has an insulating property and is thermally decomposed at a temperature of 200.degree. to 300.degree. C. and volatilized to prevent short-circuiting and the like. However, it is difficult to obtain resins having such properties, and further in a case of polyethylene terephthalate, a continuous bonding cannot be carried out due to wire break caused when a second bond is subjected to contact bonding, because it is a relatively brittle crystalline material, or in a case of polycarbonate, a bonding wire obtained after it has been coated with a film is naturally curled and thus it is difficult to process it by a high speed bonding machine and keep the positional accuracy thereof when it is processed at a high speed. As a result, at present, these materials are not practically used because of such a drawback that the workability of a bonding process is deteriorated, and the like.
Further, a new problem in practical use arises in that some of polymer resin materials used for coating a bonding wire is left between the bonding wire and a lead frame or bonding pads of a semiconductor pellet, or a piece of the coated resin broken by an impact produced when bonding is carried out cuts the bonding wire of gold or the like or injures it to cause wire cutting-off.
In general, a heat resistant insulation coating material includes resins called engineering plastics, and among them aromatic polyester has been found to be excellent in these characteristics [for example, Japanese Patent Kokai (Laid-Open) Nos. Sho 54-138056 and Sho 58-137905].
Nevertheless, generally known aromatic polyester resins are excellent in heat resistance, wear resistance, fire retardance, mechanical characteristics such as tensile strength and the like, impact resistance, and electric characteristics such as insulating property and the like, but it is poor in an extrusion processing property, and thus it has been very difficult to form a thin insulation film of uniform thickness on the circumference of a bonding wire by an electric wire coating method using an extruder. In addition, aromatic polyester resins whose extrusion processing property has been improved by modification or the like have a problem in that they have poor heat and wear resistances.
Further, there has been employed a method by which a resin material is dissolved in a proper solvent and coated to form an insulation film of uniform thickness on the circumference of a bonding wire. Since, however, aromatic polyester resins having high mechanical strength and heat resistance have high chemical resistance, a suitable solvent for it is not available and then it has been difficult to uniformly form a thin coating film on a bonding wire.
Further, when the aromatic resins are made to be easily dissolved in solvent by modification or the like, they are usually deteriorated in weather resistance, heat resistance and the like. Therefore, it has been very difficult to make an insulation bonding wire using aromatic polyester as a coating layer.
Further, when heat resistant thermosetting resins are used, there is a drawback in that carbides and the like of the resins are left in the under portion of balls even after they have been formed or the resins are not separated properly from metal in a bonding process of a bonding wire with a lead frame and thus the bonding strength thereof is greatly lowered.
Although Japanese Patent Applications Kokai Nos. Sho 60-224255 and Sho 62-30359 disclose that a copper wire having an oxide insulation coating film is used as a bonding wire, it is not yet practically used, because the removal of the oxide coating film in a bonding process and a continuous bonding of the wire are difficult.
Further, there is a trial to coat the surface of a bonding wire with another material and, for example, Japanese Patent Application Kokai No. Sho 62-97360 discloses to form precious or corrosion resistant metal on a copper electrode thin wire. A purpose of this case, however, is to prevent surface oxidation and not to solve the above problems.
Further, when such a resin coated insulation bonding wire is manufactured, polymer materials generally excellent in an insulating property are also excellent in heat resistance, wear resistance, fire retardance, mechanical characteristics such as tensile strength and the like, impact resistance, and electric characteristics such as insulation property and the like, but they are poor in an extrusion processing property. Therefore, it is very difficult to form a thin insulation film of uniform thickness on the circumference of a bonding wire by an electric wire coating method using an extruder.
To cope with this problem, there is a device to form an insulation coating film while bonding is carried out, and as an example thereof there is a method of supplying a sticky liquid insulation agent through a guide portion of a capillary tube, as disclosed in Japanese Patent Application Kokai No. Sho 52-79657.
This method, however, is difficult to smoothly supply the insulation agent, and further, as shown in Japanese Patent Kokai No. Sho 61-269319, a method of forming an insulation coating film by spraying an insulation material onto a bonding wire after it has been stretched, and the like cannot form a uniform coating film. Thus, these methods are not practical in the formation of a uniform thin insulation coating film on a bonding wire.